Timber replacement- structural composite lumber

Structural composite lumber (SCL) is a general term which is given currently to three types of engineered wood products: laminated veneer lumber (LVL), parallel strand lumber (PSL) and laminated strand lumber (LSL). These products are manufactured out of veneer or strand or small wood elements bonded together with structural adhesive. Wood grain in these products is aligned along the length to optimise its structural performance.

These products are an alternative to solid sawn timber and could be used as a support beam, header, joist or as columns.

These products offer better utilization of the wood fibre in comparison to sawn timber (Figure 2.1).

Figure 2.1 Wood fibre used more efficiently when is converted to the engineered wood product (Reproduced from Nelson 1997)

2.3.1.1 Laminated veneer lumber

Laminated veneer lumber (LVL) was the first SCL product which was commercially produced in the early 1960s; however, prior to that time LVL had been produced during the second world war in Europe and United States for making aeroplane propellers (Forest products laboratory, 1987).

Sawn timber LVL PSL LSL

CHAPTER 2:A REVIEW OF LITERATURE ON ENGINEERED WOOD PRODUCTS

2.3.1.1.1 Manufacturing process

LVL is currently manufactured in Finland, New Zealand and the USA. In general, the process is started by debarking the logs and then soaking them in the hot water for 24 hours. This is followed by peeling the log to veneers typically 3 to 4mm thick.

Veneers are dried and graded according to their stiffness and strength while passing through an Ultrasonic-Veneer-Tester* machine. Veneers are coated by structural adhesive and piled so that low grade veneer is placed in the centre while high grade veneers are positioned on top and bottom. Then the veneers enter a hot press where they are shaped into a solid panel (Figure 2.2).

Figure 2.2 LVL manufacturing process (Figure obtained from Nelson 1997)

An *ultrasonic veneer tester determines the physical properties of dry veneer sheets by repetitively measuring ultrasonic stress wave propagation time as the veneer under test moves through the machine at production line speeds.

2.3.1.1.2 Advantages of LVL

Predictability of performance, large range of available sizes, dimensional consistency, dimensional stability and easy treatability are the main advantages of LVL over

CHAPTER 2:A REVIEW OF LITERATURE ON ENGINEERED WOOD PRODUCTS

overcomes the natural timber defects like knots, slope of grain which often occur in logs.

The coefficient of variation in strength and stiffness for LVL only varies from 10 to 15 while it reaches 25 to 40 for solid sawn timber.

2.3.1.1.3 Disadvantages of LVL

There is a limitation on the size of log that could be used for the peeling process and some wood species are unsuitable for peeling. Compared to PSL and LSL, the pressing procedure does not improve the strength and stiffness significantly by increasing the density of the veneers. Finally, laminated veneer lumber is susceptible to copping across the width if stored improperly.

2.3.1.2 Parallel strand lumber

The EWP known as parallel strand lumber or PSL is the result of years of research (1971-1987) by Derek Barnes, Mark Churchland and Walter Schilling in MacMillan Bloedel ltd. of Vancouver, Canada. After MacMillan Bloedel’s and Trus Joist formed a joint venture in 1991, the technology transferred to Trus Joist MacMillan (TJM) for its management. At present PSL is produced at plants in Vancouver, British Columbia, Canada; Colbert, Georgia and Buchannan, West Virginia in the United States.

2.3.1.2.1 Manufacturing process

This process is similar to that for LVL: logs are rotary peeled into veneer sheets with 2 to 3mm thickness. The veneers are then chipped to thin strands, which have lengths up to 2400 mm and are approximately 18 mm wide. Strands are coated with structural glue and pass through a continuous rotary belt press to build a mat of highly consistent density. An adhesive is cured by microwaves. Using this method it is possible to manufacture larger cross-sections than for LVL.

Veneers for PSL are mainly obtained from sapwood, which has higher strength compare to heartwood. Currently Douglas-fir, Western hemlock, Southern Pine and Yellow-Poplar have been used as raw materials for PSL.

CHAPTER 2:A REVIEW OF LITERATURE ON ENGINEERED WOOD PRODUCTS

2.3.1.2.2 Advantages of PSL

Parallel strand lumber has all the advantages of LVL, however unlike LVL it is not possible to assemble the strand’s base on their strength and stiffness. As an alternative PSL strength is enhanced by raising the level of density of the strands.

Acceptance of preservative and fire retardant in PSL is even better than LVL. In addition the percentage of fibre utilization in PSL is higher than in LVL and this could be enhanced by running the PSL and LVL plants at one site.

2.3.1.2.3 Disadvantages of PSL

The disadvantages are similar to those of LVL. PSL products are limited to peelable logs. In comparison to solid sawn lumber and glued laminated timber, PSL products are heavier and harsher to saws and drills. Furthermore connection must be made with metal plates and bolts instead of nails.

Parallel strand lumber, like LVL, is considered as high capital technology; and it requires a stable production process.

2.3.1.3 Laminated strand lumber

Laminated strand lumber (LSL) is the newest engineered wood product which is commercially produced. The product was developed by Trus Joist MacMillan in the USA during the early 1990s (ECO link 2001) .

To some extent, there is similarity between LSL and OSB technology; the main difference is length of the strand in LSL, which reaches 30 cm. This greater length is the principal reason for LSL’s flexural strength.

2.3.1.3.1 LSL manufacturing process

Logs of Aspen and Poplar are cut into strands up to 30 cm long and 3 cm wide. Dried strands are oriented parallel to the panel length during mat formation, which takes better advantage of the wood’s natural strength. Strands are monitored and undesirable strands are separated in advance and used as fuel for the manufacturing process. The rest of the strands enter a revolving drum where adhesive is sprayed on them. A polymeric diphenylmethane diisocyanate has been used as bonding for the

CHAPTER 2:A REVIEW OF LITERATURE ON ENGINEERED WOOD PRODUCTS

strands. Adhesive curing occurs while the panel passes through a stationary steam injection press. Panels are manufactured in 2.4×10.7-14.7 m sizes with 25 to 100 mm thickness.

2.3.1.3.2 Advantages of LSL

LSL enjoys all the advantages of LVL and PSL, but unlike them, LSL is not limited by availability of peel-able logs and by log size; raw material for LSL could be obtained from various species, from small logs and from crooked logs. Some 76% of the wood fibre is used, which is nearly double the amount utilised in solid sawn timber.

Strategic layering procedure and increasing density of strands enhances the strength and limits copping potential.

2.31.3.3 Disadvantages of LSL

The dimensional stability of LSL is not as good as LVL and PSL. The higher density of the LSL wood fibre in comparison to LVL and PSL make it more susceptible to swelling with changes in moisture content and so LSL can not be used where moisture content exceeds 19% RH (Product approval.1996). Moreover, laminated strand lumber like LVL and PSL requires high capital investment.